Multicast UDP example

Example servers join to group 224.0.0.5 and listen on port 8000. Their only
method echo returns its parameter.

Client joins group to 224.0.0.5, sends multicast request to group on port 8000
and waits for 5 seconds for responses. If some responses are received,
protocol callbacks with tuple of results and individual parts are checked for
errors. If no responses are received, protocol errbacks with TimeoutError.

Because there is no common way to determine number of peers in group,
MsgpackMulticastDatagramProtocol always wait for responses until waitTimeout
expires.